The purpose of this protocol is to comprehensively evaluate patients with homozygous FH to determine their potential suitability for the gene therapy protocol. Homozygous FH is a serious genetic disease caused by mutations in the lDS receptor gene and results in markedly elevated lDL levels and severe premature conorary heart and other atherosclerotic vascular disease. In addition, a second purpose of this protocol is to provide important information regarding the extent and progesssion of atherosclerosis in homozygous FH using quantitative coronary angiography and specific non-invasive tests as predictors of coronary artery disease (cad) and clinical cardiac events in patients with homozygous FH. During the patient's annual evaluation in the GCRC the following tests, in addition to coronary angiography, will be performed: exercise thallium/echocardiography, cardiac echocardiography, cartoid duplex ultrasound, magnetic resonance imaging and angiography, and cardiac Ultrafast CT scanning. The usefulness of these non-invasive tests in predicting the occurence of CAD based on clinical cardiac events and coronary angiography will be studied.